The speed of several types of semiconductor wafer processing depends on a process gas active ingredient concentration or delivery rate at the region of the wafer surface and also on the substrate temperature maintained at the wafer surface. A goal of treatment systems is to provide uniform temperature and active ingredients distributions across the wafer process area. In certain circumstances it is also important to maintain control over these process parameters during a temperature ramping period of a treatment process.
Unfortunately, the heating and process gas delivery are under control of separate process tool components and depend on the geometry and construction of the processing unit. Prior art heating and gas delivery components of a wafer processing system have been limited in achieving time and surface uniformity of action. As a result, the speed at which the temperature dependent process reaction takes place can be different at different wafer locations. This phenomenon is generally referred to as process non-uniformity. The process non-uniformity can result in overexposure to plasma with the subsequent damage of the wafers and other negative effects. During stripping, the non-uniformity slows the resist cleaning speed and therefore causes a decrease of wafer throughput of the stripping process.
Most of the process volumes or chambers are basically symmetric about a center axis, as is the wafer. The processes parameters applied to the wafer such as heating and the active plasma ingredient delivery should also be symmetric about the wafer axis. Generally, the boundary conditions for gas/species distribution and for heat exchange process on the wafer surface boundary also tend to be of rotationally symmetric about the axis. Therefore the process and temperature non-uniformity may be considered as a superposition of a radial and azimuthal components.
The radial component of process non-uniformity is measured along the wafer radius. This component in particular is a result of incomplete mutual compensation of the temperature and plasma concentration across a wafer diameter. For example, a lower temperature in the center of the wafer would be needed to compensate for a higher plasma species activity at the center area of the wafer. If the heater design cannot compensate enough or overcompensates, a radial process non-uniformity results.
An azimuth component of process non-uniformity is measured as a degree of process speed deviation along a specified circumference on the wafer surface. This component of non-uniformity is mostly a result of a local asymmetry that is specific to the process chamber/gas supply system design or is produced by different tolerances in the system. For example, such asymmetry may be due to gas flow asymmetry at a chamber door zone or a difference in individual heating elements intensity or excessive tolerance of a radiant heater bulb filament position and (or) or bulb base mounting orientation.
Simultaneous achievement of both thermal uniformity and uniform active ingredient concentration to the region of the wafer over the process area can be difficult to provide. For example, process gas supply and process gas exhaust design for downstream microwave ashers can cause substantial radial and azimuth non-uniformity in active species delivery. The radial process non-uniformity component resulting from radial non-uniformity of temperature and radial non-uniformity of the active species delivery for a design that are basically symmetric with respect of the wafer axis could be mutually compensated. Developing of the heating sources with more flexibility in optimization of heat distribution over the wafer surface becomes important in achieving the required compensation.
For brevity of explanations, in the following text the terms Uniform Heating and Uniform Temperature are used to specify the degree of difference between the actual temperature and heat rate radial distribution compared to the distribution required for ideal compensation of the non-uniform radial plasma activity, mentioned above. The terms Process Uniformity and Azimuthal Uniformity of Heating and Temperature are applied in their original sense i.e. as degree of deviation from the corresponding statistically averaged values.
In U.S. Pat. No. 6,023,555 a positive effect has been claimed due to use of radiant heating apparatus that includes a plurality of bulbs each associated with two individual reflectors—an ellipsoidal shaped back reflector and an additional—tubular reflector.
The system disclosed in the '555 patent also exhibits certain problems. The elliptical reflector in combination with a tubular reflector creates strong local process speed dependence on individual parameters and space orientation for each set of bulb/reflector. This is because the radiant energy of each set is concentrated by individual reflectors in relatively narrow areas of the target positioned with respect to each set. This prior art arrangement can result in azimuthal and especially radial non-uniformity of the temperature and, therefore, processes speed non-uniformity. An additional source of non-uniformity is inconsistency of an individual reflector's shape, finish quality, position and orientation due to dimensional tolerances of the reflector's and bulb mountings.
The numerous manufacturing and assembly tolerances also include individual bulb intensity, filament spatial position, reflectors and bulbs base installation that can result in non-uniformity of temperature and therefore, processing rate.
Practice has shown that to achieve an adequate degree of process uniformity in the structure disclosed in U.S. Pat. No. 6,023,555 requires an individual selection of bulb and reflectors for the heaters in production. Additionally, the system disclosed in the '555 patent requires an individual realignment of the heaters during production process testing because of a strong correlation of process uniformity to individual bulb properties and combined mechanical tolerances resulting in difference in the bulb filament spatial positions.
The minimization of all individual bulb positioning tolerances, extending from the wafer surface through the heating chamber and heater design, to the filament spatial orientation appears to be difficult to achieve.
The random combination of tolerances for newly built heaters in some cases has caused high non-uniformity and required the whole assembly to be rejected during manufacture. An individual parts alignment and selection in this cases is not sufficient. U.S. Pat. No. 6,259,072 B1 describes a two level zone controlled radiant heating apparatus with focused reflectors. As practical experience shows, successful application of this configuration, may require the zone map development performed individually for each manufactured reflector what is acceptable only for expensive tools. In addition, power zoning during the wafer temperature ramp up is evidence of inefficient power usage for heating. Instead of optical redistribution of excessive heat to areas with lower wafer temperature, part of the available energy is simply not used because of the less than 100% zone power applied to the some of the array bulbs.
This is a natural problem of any pan-shaped one-wall asymmetric design where the radiant energy directed to the center axis is not under designer control i.e. energy is not redirected to the proper area of the wafer to improve uniformity.
Another problem with tubular or pan-shaped reflectors is loss of part of the bulb energy which is directed parallel or close to parallel to the wafer plane. This part of the radiant energy is absorbed by other bulbs and numerous back and force reflections with partial absorbtion on the reflector surfaces before the radiant energy leave the heater in the wafer direction. The total heating effectiveness of a such an array drops due to the absorbed energy lost without heating the wafer.
Analysis of the equipment built in conformity with the '072 patent shows that the principal effect of the part-to-part deviation is azimuth temperature non-uniformity. It is similarly exaggerated at the wafer area across each ring of bulbs due to use of separately focused reflectors for each ring of bulbs.
Note, that for each individual bulb there is an effect of non-uniformity amplification because of the focusing effect of each individual bulb in a direction across the parabolic areas of the reflector surface.
In these prior art systems, the heating elements are positioned by plugging in electrical sockets, which are in turn attached to the reflector assembly. Additional to these tolerances that generate heating non-uniformity, is a bulb base to electrical socket position tolerance.